With regard to conventional writing instruments of this type, for example, Patent Literature 1 discloses a detent structure for a clip-equipped push member of a writing instrument. The writing instrument includes: a barrel; a writing element that is slidable in the barrel; operative means that enables the sliding of the writing element from a rear side of the writing element; and the clip-equipped push member that operates the operative means from an outer side of a rear end of the writing instrument. In the writing instrument, detent means is applied to an outer wall of a tubular main body of the push member and an inner wall of the barrel to prevent the push member from rotating with respect to the barrel. Patent Literature 1 also discloses a configuration of detent means including a locking groove formed in an inner wall of a rear end of the barrel and a locking projection formed on an outer wall of the push member and a configuration of detent means including a locking projection formed on the inner wall of the rear end of the barrel and a locking groove formed in the outer wall of the push member.